This application claims the priority of German application 101 57 461.4, filed Nov. 23, 2001, the disclosure of which is expressly incorporated by reference herein.
The prior art (German publication DE 44 322 72 C2) has disclosed a refrigeration-generating system for the air-conditioning of vehicles, which comprises a compressor, a gas cooler or condenser, an internal heat exchanger, a restrictor device and an evaporator, which are connected in series and form an integral, closed vapor-compression circuit for supplying refrigeration and/or heat. A pressure which is supercritical with respect to the critical pressure of the refrigerant selected is generated on the high-pressure side of the vapor-compression circuit, and at the same time a subcritical pressure is produced on the low-pressure side of the circuit. Thermal energy is supplied to the refrigerant, which has been cooled on the low-pressure side, via the evaporator (refrigeration is dissipated). In current refrigerant circuits, the refrigerant which comes out of the evaporator enters the compressor either as superheated or saturated vapor at a low pressure, and it is then compressed to a higher pressure and a higher temperature through the supply of compressor work. The refrigerant is firstly liquefied in the condenser. The liquefaction pressure which is established, for a given condenser, is dependent on the refrigerant mass flow, i.e. on the uncompressed hot-gas mass flow and its temperature, and also the temperature of the cooling-air mass flow which dissipates the heat of condensation. When the condensation has ended, design measures enable the refrigerant to be cooled to below its condensation point. Supercooling does not always occur in a defined way in vehicle condensers, on account of the difficult thermal and hydraulic boundary conditions.
The refrigerant mass flow in the circuit is controlled by adjusting various parameters, in particular the torque and rotational speed, in the compressor. The compressor torque required is generally provided by the driving engine of the motor vehicle. With a view to achieving consumption-optimized operation of the motor vehicle driving engine, there is a need or a desire for the torque output by the driving engine to be kept at a low level, with the result that it may be the case that not every desired torque is available in every operating state. Accordingly, the maximum possible refrigerant mass flow, i.e. the permitted compressor torque, is not available at every engine operating point.
It is stated in European publication EP 1 095 804 A2 that a mass flow-controlled compressor for vehicles is realized by means of its pressure upstream and downstream of the compression and an intermediate pressure level in the compressor. The solution described in this document is based on the use of a pressure-difference control valve in the compressor. The valve controls the refrigerant mass flow as a function of the pressure difference at the compressor and the refrigerant density. This is disadvantageous in that, on account of the inaccuracy and hysteresis in the switching valve, a refrigerant mass flow of which the actual value is subject to a not insignificant deviation from the set value is established, and consequently the compressor torque can only be determined to a very unsatisfactory degree. Moreover, the pressure-difference control valve is of complex design.
One object of this invention is to provide a method for operating a refrigerant circuit. Another object is to provide a method for operating a motor vehicle driving engine. Still another object is to provide a particular refrigerant circuit.
According to one aspect of the invention, a refrigerant circuit of an air-conditioning system of a motor vehicle has a refrigerant compressor for compressing substantially gaseous refrigerant, a cooler, which is connected downstream of the refrigerant compressor, for dissipating heat from the refrigerant, a restrictor means for expanding the refrigerant, and an evaporator for transferring heat to the refrigerant. Pressure in the refrigerant circuit is measured firstly on a high-pressure side by way of at least one pressure-measuring device, and secondly on the low-pressure side by way of the at least one pressure-measuring device. The refrigerant is almost completely liquefied in the refrigerant circuit upstream of the restrictor means.
Approximately complete liquefaction of the refrigerant occurs upstream of the restrictor means (expansion valve). Complete liquefaction means that it is possible to achieve a state of the refrigerant which can be recorded unambiguously. This allows accurate and reliable determination of the refrigerant mass flow by means of thermodynamic variables, such as pressure and temperature, at the restrictor means. It is preferable for the refrigerant to be at least slightly supercooled in order to achieve complete liquefaction. The invention allows accurate determination of the refrigerant mass flow. Together with the knowledge of the compressor rotational speed and the compressor design, it is possible to calculate the compressor torque with sufficient accuracy for torque communication with the driving engine of the vehicle. It is also possible to satisfy torque stipulations of the driving engine by setting a refrigerant mass flow. Furthermore, the refrigeration circuit can be controlled as required by utilizing the knowledge of the output provided in the refrigeration system. If the air temperature in the interior of the vehicle, which is relevant to comfort and safety, is too low, the output, i.e. the circulating refrigerant mass flow, is reduced. If the temperature in the interior of the vehicle is too high, the circulating refrigerant mass flow is increased. The variation is effected by changing the refrigerant mass flow in the compressor. The temperature of the evaporator can be determined by means of the determined and therefore known pressure downstream of the restrictor means, and it is therefore possible to reliably prevent the temperature from dropping below the freezing point, leading to icing. This makes it simple to achieve an output or comfort control of the refrigeration circuit by using knowledge of the refrigerant mass flow. As a result, there is no need to measure the temperature of the air downstream of the evaporator. This results in a high potential cost saving.
According to another aspect of the invention, the refrigerant mass flow of the refrigerant circuit is used to determine the power consumed by the air-conditioning system and the torque consumed by the air-conditioning system, and furthermore to determine the power consumed by the further consumers and the torques consumed by the further consumers. The power output by the driving engine and/or the torque output by the driving engine are/is then set as a function of the requirements of the air-conditioning system and of the consumers. This allows the torque of the driving machine to be managed according to the demands of the consumers, in particular according to the demand of the air-conditioning system. As a result, inexpensive and environmentally friendly vehicles can be operated in a targeted and user-friendly manner which is safe in traffic.
Expedient configurations of the invention are apparent from the claims, and the invention is explained in more detail on the basis of various circuit diagrams and drawings.